My dog, a setter, gets that way with pheasants sometimes. What I do is take a dead bird and let her play with it a little bit at home, where she's a little more comfortable - that seems to do the trick next time we go hunting. (Don't let them eat it, just play with it a while)

I think that she gets hit with the head or scratched with the claw (not with duck obviously) from time to time and she gets a little nervous.